At present, in digital cordless communication systems which are based on the Bluetooth Standard Version 1.1, data is transmitted as standard at rates of 1 Mbit/s. In this case, a two-value GFSK modulation method (Gaussian Frequency Shift Keying) is used. The GFSK modulation method is a frequency-shift keying modulation method (FSK—Frequency Shift Keying). In the case of GFSK-based modulation, a Gaussian filter is also used at the transmission end, in order to limit the frequency bandwidth. A filter such as this results in pulse-shaping of the frequency and data pulses, with the pulse per symbol extending over a time of more than only the symbol time duration T.
One possible way to achieve higher data transmission rates is to use modulation methods with more values, such as the four-value DQPSK method (Differential Quadrature Phase Shift Keying) or, in general, the DMPSK method, in which an M-value symbol where M≧4 is transmitted instead of a two-value bit. For future versions of the Bluetooth Standard (possibly even from Version 1.2, but at the latest from Version 2.0), it is planned to increase the data rate using a modulation method with more values.
In order to achieve an increase in the data rate in later versions of a Standard for standardized digital radio transmission systems, it is worthwhile changing from a modulation method with a small number of values (for example GFSK) to a modulation method with more values (for example DQPSK) once the radio link has been in existence for a certain time. This allows backward compatibility of the new version of the Standard with the earlier versions of the Standard. Setting up a connection, or setting up a so-called piconetwork in the case of the Bluetooth Standard, can in this case first of all be carried out using the modulation method with a small number of values as used for all the appliances according to the Standard. If both of the appliances in a link or piconetwork that has been set up are designed for modulation with more values, this modulation can be used for the subsequent data transmission. In general, in digital TDMA (Time Division Multiple Access)-based mobile radio systems, the information is transmitted in the form of a data burst with a defined time. In the case of packet-oriented mobile radio systems, a data packet to be transmitted extends over one or more data bursts. A data burst comprises a first data burst header or data packet header. The header contains necessary information for addressing the remote end and for indication of the packet type, and should thus, for compatibility reasons, be transmitted using a modulation method with a small number of values, for all versions of the Standard. In particular, it is also feasible for the header to indicate to the respective remote end that it should switch to a second modulation method, with more values. Switching to a modulation method with more values then does not take place until a second part of the data burst. If a plurality of data packets are transmitted successively, the modulation method is thus switched alternately a plurality of times. For receiver-end recovery of the data that is modulated with more values in the second part of a burst, it is fundamentally possible because of the greater disturbance sensitivity involved with this to use methods which require channel estimation. The aim of channel estimation is to indicate channel parameters which describe the transmission behavior of the channel. In this case, the channel parameters include the influences of the air interface, which frequently has frequency selectivity and multipath propagation. Furthermore, it is possible to take account of the influences of transmission and/or receiving components in the channel estimation. These are frequently dependent on the modulation type being used. Furthermore, the channel parameters are also influenced by temperature effects, ageing or component tolerances of the analogue receiving components (front end).
For channel estimation, a received signal in a training sequence is generally compared with a reference signal which is known at the receiver end. The achievable estimation accuracy for channel estimation and thus also the performance of the receiver are generally increased with the number of known data elements.
Future versions of the Bluetooth Standard will provide a training sequence for channel estimation in the second part of a data burst, which is modulated with more values. However, the number of symbols in this training sequence is relatively small, so that the achievable estimation accuracy of the channel parameters determined on the basis of this training sequence may be inadequate.